Settable, form-filling loss circulation control compositions comprising in situ foamed non-hydraulic sorel cement systems and method of use

ABSTRACT

This document relates to settable, non-hydraulic foamed cement compositions comprising nitrogen gas-generating compositions used for loss circulation control.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims priority toU.S. application Ser. No. 16/137,962, now issued as U.S. Pat. No.10,287,481 on May 14, 2019, which is a divisional of U.S. patentapplication Ser. No. 15/879,169, now issued as U.S. Pat. No. 10,150,905on Dec. 11, 2018, and which is hereby incorporated by reference inentirety.

TECHNICAL FIELD

This document relates to settable, non-hydraulic foamed cementcompositions comprising nitrogen gas-generating compositions used forloss circulation control.

BACKGROUND

Natural resources such as gas, oil, and water in a subterraneanformation are usually produced by drilling a well bore down to asubterranean formation while circulating a drilling fluid in thewellbore. Fluids used in drilling, completion, or servicing of awellbore can be lost to the subterranean formation while circulating thefluids in the wellbore. In particular, the fluids may enter thesubterranean formation via depleted zones, zones of relatively lowpressure, loss circulation zones having naturally occurring fractures,weak zones having fracture gradients exceeded by the hydrostaticpressure of the drilling fluid, and so forth.

One of the contributing factors may be lack of precise information onthe dimensions of loss circulation areas, which can range frommicrofractures to vugular zones. Depending on the extent of fluid volumelosses, loss circulation is classified as seepage loss, moderate loss,or severe loss. For oil-based fluids, losses of 10-30 barrels per hourare considered moderate, and losses greater than 30 barrels per hour areconsidered severe. For water-based fluids, losses between 25 and 100barrels are considered moderate, and losses greater than 100 barrels areconsidered severe. For severe losses, the dimensions of the losscirculation zones cannot be estimated which makes it difficult to designloss circulation treatment pills based on the sized particles. Therevenue loss due to loss circulation materials (LCM) problems extendsinto tens of millions of dollars.

Loss circulation treatments involving various plugging materials havebeen used to prevent or lessen the loss of fluids from wellbores. Theideal loss circulation treatment solution will have to be adaptable toany dimension or shape of the loss circulation zone. Thus, there is aneed for a composition that can form-fill upon placement, irrespectiveof the shape and size of the thief zone.

SUMMARY

Provided in this disclosure is a non-hydraulic, foamed cementitiouscomposition that includes magnesium oxide (MgO); a salt selected fromthe group consisting of magnesium chloride (MgCl₂), magnesium sulfate(MgSO₄), ammonium hydrogen phosphate (NH₄H₂PO₄), and hydrates thereof; anitrogen gas-generating compound; and a foam surfactant.

In some embodiments, the salt is magnesium chloride hexahydrate(MgCl₂.6H₂O).

In some embodiments, the foam surfactant is selected from the groupconsisting of an alkyl sulfate salt with a C₁₂-C₁₄ carbon chain, abetaine, a hydroxysultaine and any combination thereof. In someembodiments, the foam surfactant is cocoamidopropyl hydroxysultaine.

In some embodiments, the nitrogen gas-generating compound is an azocompound. In some embodiments, the azo compound is azodicarbonamide. Insome embodiments, the azo compound is about 1% to about 10% by weight ofthe MgO.

In some embodiments, the composition includes an amine activatorselected from the group consisting of carbohydrazide (CHZ),tetraethylenepentamine (TEPA), and hydrazine sulfate. In someembodiments, the weight ratio of the azo compound to the amine activatoris about 5:1 to about 1:5. In some embodiments, the amine activator isCHZ.

In some embodiments, the composition includes an oxidizer.

In some embodiments, the composition includes a set retarder selectedfrom the group consisting of hexametaphosphate, sodium borate, sodiumcitrate, citric acid, and an aminophosphonate.

In some embodiments, the composition includes a viscosifier.

In some embodiments, the pH of the final foamed cementitious compositionis greater than about 4.

Also provided in this disclosure is a non-hydraulic, foamed cementitiouscomposition that includes magnesium oxide (MgO); a salt selected fromthe group consisting of magnesium chloride (MgCl₂), magnesium sulfate(MgSO₄), ammonium hydrogen phosphate (NH₄H₂PO₄), and hydrates thereof; ahydrazide or a semi-carbazide; an oxidizer; and a foam surfactant.

In some embodiments, the oxidizer is selected from the group consistingof peroxide, persulfate, percarbonate, perbromate, perborate salts ofammonium, alkali earth metals, and alkaline earth metals. In someembodiments, the weight ratio of the hydrazide or semi-carbazide to theoxidizer is about 1:0.25 to about 1:5.

In some embodiments, the composition includes a viscosifier.

Also provided herein is a method of treating a subterranean formation,for example, a lost circulation zone, the method including: a) forming afoamed cementitious composition that includes magnesium oxide (MgO); asalt selected from the group consisting of magnesium chloride (MgCl₂),magnesium sulfate (MgSO₄), ammonium hydrogen phosphate (NH₄H₂PO₄), andhydrates thereof; a nitrogen gas-generating compound; and a foamsurfactant; and b) introducing the foamed cementitious composition intothe well.

In some embodiments, the nitrogen gas-generating compound is an azocompound. In some embodiments, the azo compound is about 1% to about 10%by weight of the MgO. In some embodiments of the method, the compositionincludes an amine activator selected from the group consisting ofcarbohydrazide (CHZ), tetraethylenepentamine (TEPA), and hydrazinesulfate. In some embodiments, the weight ratio of the azo compound tothe amine activator is about 5:1 to about 1:5.

Also provided herein is a method of treating a lost circulation zonefluidly connected to a wellbore, the method including: a) forming afoamed cementitious composition that includes magnesium oxide (MgO); asalt selected from the group consisting of magnesium chloride (MgCl₂),magnesium sulfate (MgSO₄), ammonium hydrogen phosphate (NH₄H₂PO₄), andhydrates thereof; a hydrazide or a semi-carbazide; an oxidizer; and afoam surfactant; and b) introducing the foamed cementitious compositioninto the well, and subsequently into the lost circulation zone.

In some embodiments, the oxidizer is selected from the group consistingof peroxide, persulfate, percarbonate, perbromate, perborate salts ofammonium, alkali earth metals, and alkaline earth metals. In someembodiments, the weight ratio of the hydrazide or semi-carbazide to theoxidizer is about 1:0.25 to about 1:5.

DESCRIPTION OF DRAWINGS

FIG. 1 shows set times of compositions that include SHMP as a setretarder.

FIG. 2 shows heat evolution due to exothermic gas generation and cementsetting in compositions that include gas generating components.

DETAILED DESCRIPTION

Reference will now be made in detail to certain embodiments of thedisclosed subject matter. While the disclosed subject matter will bedescribed in conjunction with the enumerated claims, it will beunderstood that the exemplified subject matter is not intended to limitthe claims to the disclosed subject matter.

Values expressed in a range format should be interpreted in a flexiblemanner to include not only the numerical values explicitly recited asthe limits of the range, but also to include all the individualnumerical values or sub-ranges encompassed within that range as if eachnumerical value and sub-range is explicitly recited. For example, arange of “about 0.1% to about 5%” or “about 0.1% to 5%” should beinterpreted to include not just about 0.1% to about 5%, but also theindividual values (for example, 1%, 2%, 3%, and 4%) and the sub-ranges(for example, 0.1% to 0.5%, 1.1% to 2.2%, and 3.3% to 4.4%) within theindicated range. The statement “about X to Y” has the same meaning as“about X to about Y,” unless indicated otherwise. Likewise, thestatement “about X, Y, or about Z” has the same meaning as “about X,about Y, or about Z,” unless indicated otherwise.

As used herein, the terms “a,” “an,” or “the” are used to include one ormore than one unless the context clearly dictates otherwise. The term“or” is used to refer to a nonexclusive “or” unless otherwise indicated.The statement “at least one of A and B” has the same meaning as “A, B,or A and B.” In addition, it is to be understood that the phraseology orterminology employed in this disclosure, and not otherwise defined, isfor the purpose of description only and not of limitation. Any use ofsection headings is intended to aid reading of the document and is notto be interpreted as limiting; information that is relevant to a sectionheading may occur within or outside of that particular section.

In the methods described herein, the acts can be carried out in anyorder, except when a temporal or operational sequence is explicitlyrecited. Furthermore, specified acts can be carried out concurrentlyunless explicit claim language recites that they be carried outseparately. For example, a claimed act of doing X and a claimed act ofdoing Y can be conducted simultaneously within a single operation, andthe resulting process will fall within the literal scope of the claimedprocess.

The term “about” as used herein can allow for a degree of variability ina value or range, for example, within 10%, within 5%, or within 1% of astated value or of a stated limit of a range.

As used herein, a “cement” is a binder, for example, a substance thatsets and forms a cohesive mass with measurable strengths. A cement canbe characterized as non-hydraulic or hydraulic. Non-hydraulic cements(e.g., Sorel cements) harden because of the formation of complexhydrates and carbonates, and may require more than water to achievesetting, such as carbon dioxide or mixtures of specific saltcombinations. Additionally, too much water cannot be present, and theset material must be kept dry in order to retain integrity and strength.A non-hydraulic cement produces hydrates that are not resistant towater. If the proportion of water to a non-hydraulic cement issignificantly higher than the stoichiometric amounts of water, thecement composition will not set into a hardened material.“Stoichiometric amounts of water” is defined as the amount of waterrequired to form the structures of the final products containingspecific amounts of water, for example as water of crystallization.Hydraulic cements (e.g., Portland cement) harden because of hydration,which uses only water in addition to the dry cement to achieve settingof the cement. Cement hydration products, chemical reactions that occurindependently of the mixture's water content can harden even underwateror when constantly exposed to wet weather. The chemical reaction thatresults when the dry cement powder is mixed with water produces hydratesthat have extremely low solubility in water.

As used herein, a “cementitious composition” can refer to anon-hydraulic Sorel cement composition, which can include, in additionto water, mixtures of near stoichiometric quantities of magnesium oxideand a salt, to set. A cementitious composition can also includeadditives. The cementitious compositions described herein can includewater and/or be mixed with water. Depending on the type of cement, thechemical proportions, when a cement composition is mixed with water, itcan begin setting to form a single phase solid material.

As used herein, the term “set” can mean the process of a fluid slurrybecoming a hard solid. Depending on the composition and the conditions,it can take just a few minutes up to 72 hours or longer for some cementcompositions to initially set.

As used herein, a “retarder” can be a chemical agent used to increasethe thickening time of a cement composition. The need for retarding thethickening time of a cement composition tends to increase with depth ofthe zone to be cemented due to the greater time required to complete thecementing operation and the effect of increased temperature on thesetting of the cement. A longer thickening time at the designtemperature allows for a longer pumping time that may be required.

The term “alkyl” as used herein can refer to straight chain and branchedalkyl groups and cycloalkyl groups having from 1 to about 40 carbonatoms, 1 to about 20 carbon atoms, 1 to about 12 carbons or, in someembodiments, from 1 to about 8 carbon atoms. Examples of straight chainalkyl groups include those with from 1 to about 8 carbon atoms such asmethyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, andn-octyl groups. Examples of branched alkyl groups include, but are notlimited to, isopropyl, iso-butyl, sec-butyl, t-butyl, neopentyl,isopentyl, and 2,2-dimethylpropyl groups. As used herein, the term“alkyl” encompasses n-alkyl, isoalkyl, and anteisoalkyl groups as wellas other branched chain forms of alkyl. Representative substituted alkylgroups can be substituted one or more times with any of the groupslisted herein, for example, amino, hydroxy, cyano, carboxy, nitro, thio,alkoxy, and halogen groups.

The term “amine” as used herein can refer to primary, secondary, andtertiary amines having, e.g., the formula N(group)₃ wherein each groupcan independently be H or non-H, such as alkyl, aryl, and the like.Amines include, but are not limited to, RNH₂, for example, alkylamines,arylamines, alkylarylamines; R₂NH wherein each R is independentlyselected from, for example, dialkylamines, diarylamines, aralkylamines,heterocyclylamines and the like; and R₃N wherein each R is independentlyselected from, for example, trialkylamines, dialkylarylamines,alkyldiarylamines, triarylamines, and the like.

The term “amino group” as used can herein refer to a substituent of theform —NH₂, —NHR, —NR₂, wherein each R is independently selected, andprotonated forms of each. Accordingly, any compound substituted with anamino group can be viewed as an amine. An “amino group” within themeaning herein can be a primary, secondary, or tertiary amino group. An“alkylamino” group includes a monoalkylamino, dialkylamino, andtrialkylamino group.

The term “room temperature” as used herein can refer to a temperature ofabout 15° C. to about 28° C.

As used herein, the term “polymer” can refer to a molecule having atleast one repeating unit and can include copolymers.

The term “downhole” as used herein can refer to under the surface of theearth, such as a location within or fluidly connected to a wellbore.

As used herein, the term “fluid” can refer to liquids and gels, unlessotherwise indicated.

As used herein, the term “drilling fluid” can refer to fluids, slurries,or muds used in drilling operations downhole, such as during theformation of the wellbore.

As used herein, the term “cementing fluid” can refer to fluids orslurries used during cementing operations of a well. For example, acementing fluid can include an aqueous mixture including at least one ofcement and cement kiln dust. In another example, a cementing fluid caninclude a curable resinous material such as a polymer that is in an atleast partially uncured state.

As used herein, the term “subterranean material” or “subterraneanformation” can refer to any material under the surface of the earth,including under the surface of the bottom of the ocean. For example, asubterranean formation or material can be any section of a wellbore andany section of a subterranean petroleum- or water-producing formation orregion in fluid contact with the wellbore. Placing a material in asubterranean formation can include contacting the material with anysection of a wellbore or with any subterranean region in fluid contacttherewith. Subterranean materials can include any materials placed intothe wellbore such as cement, drill shafts, liners, tubing, casing, orscreens; placing a material in a subterranean formation can includecontacting with such subterranean materials. In some examples, asubterranean formation or material can be any below-ground region thatcan produce liquid or gaseous petroleum materials, water, or any sectionbelow-ground in fluid contact therewith. For example, a subterraneanformation or material can be at least one of an area desired to befractured, a fracture or an area surrounding a fracture, and a flowpathway or an area surrounding a flow pathway, wherein a fracture or aflow pathway can be optionally fluidly connected to a subterraneanpetroleum- or water-producing region, directly or through one or morefractures or flow pathways.

As used herein, “treatment of a subterranean formation” can include anyactivity directed to extraction of water or petroleum materials from asubterranean petroleum- or water-producing formation or region, forexample, including drilling, stimulation, hydraulic fracturing,clean-up, acidizing, completion, cementing, remedial treatment,abandonment, and the like.

As used herein, a “flow pathway” downhole can include any suitablesubterranean flow pathway through which two subterranean locations arein fluid connection. The flow pathway can be sufficient for petroleum orwater to flow from one subterranean location to the wellbore orvice-versa. A flow pathway can include at least one of a hydraulicfracture, and a fluid connection across a screen, across gravel pack,across proppant, including across resin-bonded proppant or proppantdeposited in a fracture, and across sand. A flow pathway can include anatural subterranean passageway through which fluids can flow. In someembodiments, a flow pathway can be a water source and can include water.In some embodiments, a flow pathway can be a petroleum source and caninclude petroleum. In some embodiments, a flow pathway can be sufficientto divert from a wellbore, fracture, or flow pathway connected theretoat least one of water, a downhole fluid, or a produced hydrocarbon.

Compositions and Reaction Products Thereof

Provided in this disclosure are settable, non-hydraulic cementcompositions comprising nitrogen-gas generating compositions. The foamedcementitious compositions can form-fill upon placement, irrespective ofthe shape and size of the thief zone, to cure loss circulation problems.The foamed compositions can set up to hard masses to withstandhydrostatic pressures from wellbore fluids without requiring extensivefoam equipment that can involve cryogenic nitrogen and the associatedmachinery. The compositions described herein are in situ foamingcompositions that include fast-setting Sorel cement compositions. Sorelcement compositions typically contain magnesium oxide and a solublemagnesium salt, such as magnesium chloride and magnesium sulfate, or aphosphate salt such as sodium or ammonium hydrogen phosphate. Suchcements are individually referred to as magnesium oxychloride (MOC),magnesium oxysulfate (MOS) and magnesium oxyphosphate (MOP) cementsystems, and collectively as Sorel cements. Provided herein are Sorelcement-based slurries that are foamed with in situ generated nitrogengas. The gas is generated by a nitrogen gas-generating compound. In someembodiments, the compositions comprise an activator or acceleratorcompound that can accelerate generation of gas from the gas-generatingcompound.

Provided in this disclosure is a foamed cementitious compositionincluding magnesium oxide (MgO), a salt, a nitrogen gas-generatingcompound, and a foam surfactant. Also provided in this disclosure is afoamed cementitious composition that includes magnesium oxide, a salt, ahydrazide or semi-carbazide, an oxidizer, and a foam surfactant.

In some embodiments, the non-hydraulic foamed cement compositionsdescribed herein include Sorel cements which are a combination ofmagnesium oxide, a magnesium salt, and water. Sorel cement (also knownas magnesia cement) is a non-hydraulic cement that is typically amixture of magnesium oxide (burnt magnesia) and a magnesium salt, suchas magnesium chloride, that when mixed with water hardens and sets.Without being limited by any theory, it is believed that the mainproducts formed in Sorel cements based on magnesium chloride andmagnesium oxide include magnesium hydroxide (Mg(OH)₂), a 3-formmagnesium oxychloride of the composition 3Mg(OH)₂. MgCl₂.8H₂O, and a5-form magnesium oxychloride product of the composition 5Mg(OH)₂.MgCl₂.8H₂O. The 5-form product has superior mechanical properties and isthe primary product formed when the molar ratio of its componentsMgO:MgCl₂:H₂O is about 5:1:13, when there is a slight excess of MgO andan amount of water required to form the 5-form and to convert any excessMgO into Mg(OH)₂. For the 3-form, the molar ratio of MgO:MgCl₂:H₂O is3:1:11.

The magnesium salts in the foamed cementitious compositions describedherein can include, for example, magnesium chloride (MgCl₂) or magnesiumsulfate (MgSO₄). In some embodiments, the Sorel cement compositionsdescribed herein include compositions containing magnesium oxide, asodium or ammonium hydrogen phosphate, and water.

In some embodiments, the salt is a magnesium salt, a sodium phosphatesalt, an ammonium phosphate salt, or hydrates thereof. In someembodiments, the salt is a magnesium salt or hydrate thereof selectedfrom magnesium chloride (MgCl₂) and magnesium sulfate (MgSO₄). In someembodiments, the salt is magnesium chloride or a magnesium chloridehydrate with the formula MgCl₂ (H₂O)_(x). In some embodiments, the saltis magnesium chloride hexahydrate, with the formula MgCl₂ (H₂O)₆ orMgCl₂.6H₂O. In some embodiments, the salt is magnesium sulfate. In someembodiments, the salt is a sodium phosphate salt selected from among amonosodium phosphate salt and a disodium phosphate salt. In someembodiments, the salt is monosodium dihydrogen phosphate. In someembodiments, the salt is an ammonium dihydrogen phosphate salt.

In some embodiments, the molar ratio of magnesium oxide to soluble saltis between about 1:0.5 to 6:1.

In some embodiments, the nitrogen gas-generating compound is selectedfrom among an azo compound, a hydrazide, a semi-carbazide, andcombinations thereof. In some embodiments the azo compound is aderivative of azodicarboxylic acid with the formula:

where X is independently selected from among NH₂, a monoalkylaminogroup, a dialkylamino group, OH, O⁻M^(n+) (where M^(n+) is an alkali oralkaline earth metal), alkyl, aryl, or an alkoxy group. In someembodiments, the azodicarboxylic acid derivative is selected from amongan amide derivative, an ester derivative, and an alkali salt of thecarboxylic derivative. In some embodiments, the nitrogen gas-generatingazodicarboxylic acid derivative is azodicarbonamide (AZDC) with thestructure:

In some embodiments, the nitrogen gas-generating azodicarboxylic acidderivative is an ester selected from among diisopropyl azodicarboxylate(DIAD) and diethyl azodicarboxylate (DEAD) represented by thestructures:

In some embodiments, the azo compound is toluene sulfonyl hydrazide withthe structure:

In some embodiments, the nitrogen gas-generating compound is a hydrazidewith the structure:

In some embodiments, the hydrazide is carbohydrazide (CHZ) and R isNHNH₂. In some embodiments, the hydrazide is p-toluenesulfonylhydrazide.

In some embodiments, the nitrogen gas-generating compound is asemi-carbazide with the structure:

In some embodiments, the semi-carbazide is an unsubstitutedsemi-carbazide and R is H (i.e., hydrazinecarboxamide).

In some embodiments, the compositions described herein include an azocompound and a hydrazide. In some embodiments, the compositionsdescribed herein include AZDC and CHZ.

In some embodiments, the foamed cementitious composition includes anitrogen gas-generating compound in an amount of about 0.1% to about 20%by weight of the MgO. For example, the nitrogen gas-generating compoundcan be about 1% to about 10% by weight of the MgO or about 0.5%, 1%,1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%,8.5%, 9%, 9.5%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, orabout 20% by weight of the MgO.

In some embodiments, the foam surfactant is selected from among an alkylsulfate salt, an alpha-olefin sulfonate, a betaine, a hydroxysultaine,and an amine oxide, and combinations thereof. In some embodiments, thealkyl sulfate salt has an alkyl chain that is a C₁₂-C₁₄ carbon chain,such as sodium dodecyl sulfate. In some embodiments, the foam surfactantis cocoamidopropyl hydroxysultaine. In some embodiments, the foamsurfactant is a combination of alkyl sulfate salt and cocoamidopropylhydroxysultaine.

In some embodiments, the foam surfactant is present in an amount ofabout 30 wt % to about 50 wt % in an aqueous solution. In someembodiments, the foam surfactant is in an aqueous solution containing awater soluble alcohol, for example isopropyl alcohol. In someembodiments, the foam surfactant is about 30% to about 50% by weight inthe aqueous solution. For example, the foam surfactant can be about 30%,35%, 40%, 45%, or about 50% by weight in the aqueous solution. In someembodiments, the foam surfactant is a 44 wt % solution ofcocoamidopropyl hydroxysultaine in water.

In some embodiments, the surfactant or combination of surfactants isadded in about 1% to about 10% by volume of the mix water used to makethe cement composition. In some embodiments, the surfactant solution isadded in about 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or about 10% by volume ofthe mix water used to make the cement composition.

In some embodiments, the foamed cementitious composition includes anamine activator or accelerator compound. The amine activator/acceleratorcompound can be used to accelerate the generation of gas from thenitrogen gas-generating compound. In some embodiments, the amineactivator is selected from among a hydrazide, a hydrazine, and anethyleneamine. Examples of suitable ethyleneamines includeethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine(TETA), and tetraethylenepentamine (TEPA). In some embodiments, theamine activator is TEPA. In some embodiments, the hydrazine is ahydrazine salt. In some embodiments, the amine activator is hydrazinesulfate. In some embodiments, the amine accelerator compound is ahydrazide with the structure:

In some embodiments, the hydrazide is carbohydrazide (CHZ) and R isNHNH₂. In some embodiments, the amine activator compound is asemi-carbazide with the structure:

In some embodiments, the semi-carbazide is an unsubstitutedsemi-carbazide and R is H (i.e., hydrazinecarboxamide). In someembodiments, the hydrazide is p-toluenesulfonyl hydrazide. In someembodiments, the compositions described herein include an azo compoundand a hydrazide. In some embodiments, the amine activator iscarbohydrazide (CHZ). In some embodiments, the composition comprises anitrogen gas-generating compound that is an azo compound and an amineactivator. In some embodiments, the composition comprises AZDC and CHZ.In some embodiments, the composition comprises AZDC and TEPA. In someembodiments, the composition comprises AZDC and hydrazine sulfate.

In some embodiments, the weight ratio of the nitrogen gas-generatingcompound to the amine activator is about 20:1 to about 1:20, such asabout 10:1 to about 1:10, or about 5:1 to about 1:5. For example, theweight ratio of the nitrogen gas-generating compound to the amineactivator can be about 5:1 to about 1:5, about 5:1 to about 1:4, about5:1 to about 1:3, about 5:1 to about 1:2, about 5:1 to about 1:1, orabout 5:1, 4:1, 3:1, 2:1, 1:1, 1:2, 1:3, 1:4, or about 1:5. In someembodiments, the nitrogen gas-generating compound is an azo compound. Insome embodiments, the azo compound is AZDC.

In some embodiments, the foamed cementitious composition includes anoxidizing compound (oxidizer). In some embodiments, the oxidizingcompound is selected from among a peroxide, a persulfate, apercarbonate, a perbromate, a perborate salt of ammonium, an alkaliearth metal, and an alkaline earth metal. In some embodiments, theoxidizing compound is selected from among potassium persulfate, sodiumpersulfate, magnesium peroxide, encapsulated potassium persulfate, andencapsulated potassium bromate compounds. In some embodiments, theoxidizing compound is potassium persulfate. In some embodiments, theoxidizing compound is magnesium peroxide. In some embodiments, theoxidizing compound is encapsulated potassium persulfate. In someembodiments, the amine activator can generate additional nitrogen bythemselves in the presence of an oxidizing compound. In someembodiments, carbohydrazide can generate additional nitrogen in thepresence of an oxidizing compound. In some embodiments, hydrazinesulfate can generate additional nitrogen in the presence of an oxidizingcompound. In some embodiments, the composition comprises CHZ andpotassium persulfate. In some embodiments, the composition comprises CHZand magnesium peroxide. In some embodiments, the composition comprisesCHZ and encapsulated potassium persulfate.

In some embodiments, the oxidizing compound is present in an amount ofabout 0.001 wt % to about 2 wt % of the magnesium oxide, or about 0.005wt %, 0.01 wt %, 0.05 wt %, 0.1 wt %, 0.5 wt %, 1 wt %, 1.5 wt %, orabout 2 wt % of the magnesium oxide. In some embodiments, the oxidizeris used in combination with a carbazide, a hydrazide, a semi-carbazideand hydrazine sulfate.

In some embodiments, the foamed cementitious composition includes a setretarder. In some embodiments, set times of Sorel cements at a giventemperature can be controlled by set retarders. In some embodiments, theset retarder is selected from among a citrate salt, citric acid, sodiumhexametaphosphate, aminomethylene organophosphonates, and sodium boratesalts. In some embodiments, the set retarder is selected from amongsodium hexametaphosphate (SHMP), sodium borate, sodium citrate, citricacid, sodium tetraborate and the pentasodium salt of amino tri(methylenephosphonic acid) (Na₅ATMP). An exemplary Na₅ATMP salt includes Dequest2006®, available as a 40% solution from Italmatch Chemicals (Red Bank,N.J.). In some embodiments, the set retarder is SHMP.

In some embodiments, the set retarder is present in an amount of 0.5 wt% to about 10 wt % of the magnesium oxide, or about 1 wt %, 2 wt %, 3 wt%, 4 wt %, 5 wt %, 6 wt %, 7 wt %, 8 wt %, 9 wt %, or about 10 wt % ofthe magnesium oxide. The amount of retarder is determined by labexperimentation by measuring the thickening times using a conventionalequipment, such as a Cement Consistomer, at temperature and pressureconditions of the subterranean formation.

In some embodiments, the foamed cementitious composition includes aviscosifier, such as a polymeric viscosifier. In some embodiments, theviscosifier can prevent settling of the magnesium oxide. In someembodiments, the viscosifier can improve foam stability. In someembodiments, the viscosifier is selected from among xanthan, diutan andvinylphosphonic acid-grafted hydroxyethyl cellulose (HEC-VP). Anexemplary HEC-VP includes Special Plug, available as a 30 wt % polymerslurry in a non-aqueous polyol (Special Products Division of ChampionChemicals, TX). In some embodiments, the viscosifier is xanthan. In someembodiments, the viscosifier is HEC-VP. In some embodiments, theviscosifier is in an aqueous solution.

In some embodiments, the viscosifier is present in an amount of about0.5 wt % to about 5 wt % of mix water used to prepare the cementcomposition. The amount of viscosifier is determined by labexperimentation by measuring the thickening times using a conventionalequipment, such as a Cement Consistomer, at temperature and pressureconditions of the subterranean formation. In some embodiments, theviscosifier is in an aqueous solution. For example, the viscosifier canbe about 0.1% to about 5% by weight of the mix water, such as about0.1%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 2%, 2.5%, 3%, 4%, or about 5% byweight of a the mix water. In some embodiments, the viscosifier isdiutan and is about 0.5 wt % of the mix water. In some embodiments, theviscosifier is xanthan and is about 0.6 wt % of the mix water. In someembodiments, the viscosifier is HEC-VP and is about 0.9 wt % of the mixwater. In some embodiments, the mix water solution containing theviscosifier has a pH of 1.6. In some embodiments, the mix water solutioncontaining the viscosifier has a pH of about 6-7.

Additional components that can be added to the cementitious compositionsdescribed herein include dispersants, set accelerators, settlingprevention additives, water proofing chemicals such as organosiliconatesand the like, cement extender/filler materials such as flyashes, slag,silica and sand, mechanical property modifiers such as fibers, latexmaterials, and rubber particles.

In some embodiments, the pH of the final foamed cementitious compositionis about 4 or greater than about 4, for example about 4, 5, 6, 7, 8, 9,or greater. In some embodiments, the composition has a pH of greaterthan about 4 at the time the composition is placed in a well.

Also provided in this disclosure is a foamed cementitious compositionthat includes MgO, a magnesium chloride salt, azodicarbonamide, ahydroxysultaine, SHMP, xanthan, and an amine activator selected fromamong CHZ, TEPA, and hydrazine sulfate. In some embodiments, the amineactivator is CHZ. In some embodiments, the amine activator is TEPA. Insome embodiments, the amine activator is hydrazine sulfate.

Method of Treating a Subterranean Formation

Additionally, provided in this disclosure is a method of treating asubterranean formation. In some embodiments, the subterranean formationis a lost circulation zone. The method includes forming a foamedcementitious composition described herein, and introducing the foamedcementitious composition into the subterranean formation. In someembodiments, the composition includes magnesium oxide (MgO); a saltselected from among magnesium chloride (MgCl₂), magnesium sulfate(MgSO₄), ammonium hydrogen phosphate (NH₄H₂PO₄), and hydrates thereof; anitrogen gas-generating compound; and a foam surfactant; and introducingthe foamed cementitious composition into the subterranean formation.

In some embodiments, the nitrogen gas-generating compound is an azocompound. In some embodiments, the azo compound is about 1% to about 10%by weight of the MgO.

In some embodiments, the composition includes an amine activatorselected from among carbohydrazide (CHZ), tetraethylenepentamine (TEPA),and hydrazine sulfate. In some embodiments, the weight ratio of the azocompound to the amine activator is about 5:1 to about 1:5.

Also provided is a method of treating a subterranean formation, such asa lost circulation zone, that includes forming a foamed cementitiouscomposition that includes MgO; a salt selected from among magnesiumchloride (MgCl₂), magnesium sulfate (MgSO₄), ammonium hydrogen phosphate(NH₄H₂PO₄), and hydrates thereof; a hydrazide or a semi-carbazide; anoxidizer; and a foam surfactant; and introducing the foamed cementitiouscomposition into the lost circulation zone.

In some embodiments, the oxidizer is selected from among peroxide,persulfate, percarbonate, perbromate, perborate salts of ammonium,alkali earth metals, and alkaline earth metals.

In some embodiments, the weight ratio of the hydrazide, carbazide,semi-carbazide or hydrazine sulfate to the oxidizer is about 1:0.25 toabout 1:5.

The cementitious compositions described herein can be prepared by mixingthe cementitious solids with mix water which can be fresh water, seawater, or brine. The mix water can be premixed with gas generatingmaterials, retarders or other additives intended for slurry or setcement property manipulation to meet the requirements. Dry cementpowders, or blends mixed with solid additives are added to mix stirringat agitation speeds recommended by American Petroleum Instituteguidelines where appropriate. Liquid additives are injected into the mixwater or into the slurry during or after slurry preparation or whilepumping the slurry downhole. Such liquid additives may include foamingcompositions, foaming surfactants, oxidizers or retarders and the like.After placing the foamed composition in the zone of interest, thecomposition is typically allowed to set for at least 24 hours beforeconducting further operations such as drilling, cementing, or wellborecleanup.

Also provided herein is a method of servicing a wellbore. The methodincludes providing a foamed cementitious composition including magnesiumoxide (MgO); a salt selected from the group consisting of magnesiumchloride (MgCl₂), magnesium sulfate (MgSO₄), ammonium hydrogen phosphate(NH₄H₂PO₄), and hydrates thereof; a nitrogen gas-generating compound;and a foam surfactant, within a portion of at least one of a wellboreand a subterranean formation.

Also provided herein is a method of servicing a loss circulation zonefluidly connected to a wellbore that includes providing a foamedcementitious composition including magnesium oxide (MgO); a saltselected from the group consisting of magnesium chloride (MgCl₂),magnesium sulfate (MgSO₄), ammonium hydrogen phosphate (NH₄H₂PO₄), andhydrates thereof; a hydrazide or a semi-carbazide; an oxidizer; and afoam surfactant, within a portion of at least one of a wellbore and asubterranean formation containing the lost circulation zone.

In some embodiments, the composition is introduced into at least one ofa wellbore and a subterranean formation containing the lost circulationzone using a pump.

EXAMPLES

A series of magnesium chloride and magnesium oxide-based compositions,referred to as magnesium oxychloride cements (MOC), were tested asrepresentative Sorel cement compositions, and described in Examples 1-6.The compositions were prepared by adding magnesium chloride andmagnesium oxide to water along with the other ingredients in therequired stoichiometric amounts and allowed to set in a closed, dryatmosphere. Alternately, magnesium brines were mixed with requiredamounts of magnesium oxide and allowed to set.

Other components of the compositions included: azodicarbonamide; anamine activator/accelerator compound, including carbohydrazide,tetraethylenepentamine (TEPA), and hydrazine sulfate; an oxidizingcompound, including potassium persulfate, sodium persulfate, magnesiumperoxide, encapsulated potassium persulfate, and encapsulated potassiumbromate compounds; a foaming surfactant, including an alky sulfatesalts, in which the alkyl group has C12-C14 carbon chain length, abetaine and a hydroxysultaine; a set retarder, including sodiumhexametaphosphate (SHMP), sodium citrate, sodium tetraborate, andpentasodium salt of amino tri(methylene phosphonic acid) (Na₅ATMP)(Dequest 2006®, a 40% solution from Italmatch USA, Red Bank, N.J.); anda polymeric viscosifier, including xanthan, diutan, and vinylphosphonicacid-grafted cellulose (HEC-VP) (available as a 30 wt % polymer slurryin a non-aqueous polyol available from Special Products Division ofChampion Chemicals, Texas under the trade name Special Plug). The diutanand xanthan solutions were prepared as 0.5 wt % and 0.6 wt % solutions,respectively, by dissolving the polymer is water with mild agitation tominimize shear induced polymer chain scission. The Special Plug productsolution was prepared by stirring 12.5 mL of the polymer slurry in 400mL water, followed by addition of 1.25 mL concentrated hydrochloric acidto obtain a 0.9% polymer solution with a pH of 1.6.

Example 1—Cement Compositions Containing a Gas Generating Compound andan Activator

A stock solution containing 30 g magnesium chloride hexahydrate, 2 mLcocoamidopropyl hydroxysultaine (44% solution in water), and 4 g sodiumhexametaphosphate in 50 mL of a 0.8 wt % xanthan solution was prepared.To this solution, 36 g of magnesium oxide and 1.75 g of azodicarbonamidewas added with stirring. The density of the slurry was 1.46 g/cm³ (12.2pounds per gallon, ppg). The slurry was divided into four 25 g (17 mL)portions and added to Humbolt cardboard cylinders. An activator, eithercarbohydrazide (CHZ), TEPA, or hydrazine sulfate, was added to eachcylinder. The cylinders were kept in a water bath thermostated at 140°F. with no contact with water for 48 hr. The volume and gas volume ofeach sample was measured, and the results are shown in Table 1.

TABLE 1 % gas Activator Final Gas vol. in amount vol. vol. setComposition Activator (g) (mL) (mL) cement Observations 1a None 0 25 832 Unstable foam layer on top 1b Carbohydrazide 0.32 82 65 73 Uniform(CHZ) foam 1c TEPA 0.30 31 14 45 Unstable foam on top 1d Hydrazine 0.3472 55 76 Uniform sulfate foam

The results showed that amine compounds functioned as activators forazodicarbonamide in the generation of nitrogen gas.

Example 2—Cement Compositions Containing a Gas Generating Compound andan Activator

Magnesium chloride hexahydrate (60 g), 4 mL of a cocamidopropylhydroxysultaine solution (44 wt %), 3.5 g azodicarbonamide, 8 g ofsodium hexametaphosphate, and 72 g magnesium oxide were added to astirred solution of 100 mL of 0.8% xanthan to obtain a slurry with adensity of 1.49 g/cc (12.4 ppg). The slurry was divided into fourbatches and added to 2″×4″ brass molds in the amounts shown in Table 2.Separately, a 30.3 wt % aqueous solution of carbohydrazide was preparedby dissolving 1 g of carbohydrazide in 2.0 mL water and 0.3 mLconcentrated hydrochloric acid. The resulting solution contained 0.4 gcarbohydrazide per gram of the solution. Variable amounts of thecarbohydrazide solution were added to the slurries kept in the brassmolds. The molds were kept in a water bath at 140° F. and allowed to setfor 48 hrs. The volume of the set solid was measured and the amount ofgas formed was calculated. The results are shown in Table 2.

TABLE 2 AZDC Final set Final set % gas Slurry in CHZ solid solid Gasvol. vol. in vol. slurry added vol. density Formed set Composition (mL)(g) (g) (mL) (ppg) (mL) solid 2a 35 0.68 None 73 5.8^(a) 38 52 2b 420.83 0.2 180 2.9^(b) 138 77 2c 42 0.83 0.4 180 2.9^(b) 138 77 2d 43 0.850.6 160 3.3^(c) 117 73 ^(a)Firm solid; ^(b)loose unconsolidated solid;^(c)solid with mechanical integrity and uniform foaming

Example 3—Cement Composition Containing a Gas Generating Compound,Activator and Oxidizer Combination

Using a 0.8% solution of HEC-VP as the viscosifier, a solution thatcontained 151 g magnesium chloride hexahydrate in 252 g of theviscosifier solution was prepared. The pH of the solution was 0.49. Nosignificant heat increase was observed. Ten milliliters ofcocoamidopropyl hydroxysultaine (44 wt %) solution was added. Withvigorous stirring, 8.4 g AZDC was added and stirred for about 30 minuntil a uniform suspension was obtained. The pH of the suspension was0.69. To this suspension, 8.4 g carbohydrazide (CHZ) was added withstirring. The pH of the resulting suspension was 4.2, indicating thatCHZ activator increased the pH of the fluid. The density of the fluidwas 1.16 g/cm³ (9.67 ppg). To 75 mL of the suspension, 37 g of magnesiumoxide and 6.9 g of sodium hexametaphosphate was added with stirring. Gasbegan to form during the addition stage. To a second 75 mL portion ofthe suspension, 6.9 g of sodium hexametaphosphate was added and stirred.The pH of the suspension was 4.0. A mixture of 37 g magnesium oxide and1.85 g potassium persulfate were added with stirring. Both of thereaction mixtures were kept at 140° F. for 10 minutes, and the densitiesof the foamed fluids were measured. The density of the fluid without theoxidizer was 0.45 g/cc (3.75 ppg) and that of the fluid containingoxidizer was 0.34 g/cc (2.83 ppg). From the measured densities of thefoamed fluids, and the design density (1.47 g/cc or 12.3 ppg), thevolume % of the gas in the foamed fluids was calculated to be 76% forthe fluid without the oxidizer, and 81% for the fluid containing theoxidizer.

The results indicated that addition of an oxidizer increased the amountof gas generated when used in combination with AZDC and CHZ.

Example 4—Optimization of Oxidizer Structure and Concentration in CementCompositions

The following compositions were prepared by combining the components inthe order listed in Table 3 below. The amounts of gas generated weredetermined by measuring the difference between the volume of the foamedslurry and the original slurry. The mixing viscosified fluid was 0.8 wt% HEC-VP used in amounts of 7.5 mL for each test. The compositions werekept at 150° F. overnight. The results are shown in Table 3.

TABLE 3 3a 3b 3c 3d 3e 3f 3g MgCl₂•6H₂O (g) 4.5 4.5 4.5 4.5 4.5 4.5 4.5Surfactant¹ (mL) 0.3 0.3 0.3 0.3 0.3 0.3 0.3 AZDC (g) — 0.25 0.25 — —0.5 0.5 CHZ (g) 0.5 0.25 0.25 0.5 0.5 — — MgO 5   5 5 5 5 5 5 MgO₂ (g) —— — — 0.25 — — K₂S₂O₈ (g) 0.5 0.5 0.25 0.25 0.25 0.25 — Initial vol.(mL) 12   12 12 12 12 12 12 Final vol. (mL) 40   60 60 35 30 25 23 % gasvol. 70²  80 80 66 60 52 48 ¹Cocoamidopropyl hydroxysultaine ²In aseparate experiment, the gas volume in the slurry was measured to be 45%at room temperature in one hour and reached 74% in 15 min at 150° F.Similar amounts were measured when the amount of potassium persulfatewas reduced to 0.25 g.

The results in Table 3 indicated that AZDC by itself produced gasamounting to 50% by volume of the slurry when the amounts were 10% byweight of MgO. Addition of the oxidizer K₂S₂O₈ did not significantlyincrease the amounts of gas generated. Carbohydrazide, which functionsas an activator for AZDC as shown in Tables 1 and 2, generated nitrogengas by itself in the presence of the oxidizers K₂S₂O₈ or magnesiumperoxide, which indicated that the hydrazide functionality was oxidizedto nitrogen gas. The highest gas amounts were formed when a mixture ofAZDC and CHZ was used in the presence of K₂S₂O₈. The increase in gasproduction from the mixture of AZDC and CHZ in the presence of K₂S₂O₈was only slightly higher than in the absence of oxidizer, as evident bycomparison with results shown in Tables 1 and 2.

In separate experiments, the effect of encapsulation of the oxidizer onthe gas evolution rate was studied. Commercial samples of encapsulatedpotassium persulfate designed for slow and fast release rates wereobtained. Additionally, encapsulated potassium bromate was also obtainedfrom commercial sources.

Into 30 mL of a 0.8% HEC-VP solution in water, 18 g of MgCl₂.6H₂O, 1.3mL of cocoamidopropyl hydroxysultaine, 1.0 g of carbohydrazide and 1.0 gof AZDC were added and stirred vigorously for 5 minutes to obtain ahomogeneous suspension. The slurry was divided into three portions of12.8 g each and placed into graduated centrifuge tubes. A mixture of 5.0g magnesium oxide and 1.0 g of sodium hexametaphosphate was added andstirred. Into each slurry, the solid encapsulated oxidizer was added inamounts of 0.25 g, 0.5 g and 1.0 g, and the tubes were placed in oilbaths kept at 140° F. The volume increases were measured for 15 minuteswhen the gas generation was complete. The results suggested that thedegree of encapsulation of the oxidizer was insufficient to slow downthe release of the oxidizer.

Example 5—Set Retarder Selection by Exothermicity of ReactionMeasurements

Heat evolution reflected by an increase in temperature due tohydration/product formation was expected to reflect loss of fluidity ofthe slurry during placement. In a typical experiment, the slurry wasprepared by dissolving 18 g MgCl₂.6H₂O in 30 mL of an HEC-VP solution(0.8 wt % polymer). The slurry was divided into three portions of 12 gand each was placed in a centrifuge tube. Into one tube, 5 g ofmagnesium oxide was added. Into a second tube, a mixture of 5 g MgO and0.2 g sodium hexametaphosphate (SHMP, 4% by wt of MgO) was added. Intothe third tube a mixture of 5 g MgO and 1 g of SHMP (20% by wt ofMgO-labeled as 5× in FIG. 1) was added. The contents of each tube werevigorously stirred with a spatula, and the centrifuge tubes were placedin preheated, thermostated oil baths connected to Brookfieldviscometers. Thermocouples were inserted and the reaction temperatureswere monitored as a function of time under quiescent conditions.Temperature increase due to heat evolution was measured.

In one set of experiments, the effect of retarder concentration at aselected temperature was measured. In another set of experiments, theeffect of temperature on set time at a given concentration was measured.In another set of experiments, the effect of exothermic gas generationon the set times of slurries containing SHMP as the set retarder wasstudied. In this set of experiments, the slurry was prepared bydissolving 18 g MgCl₂.6H₂O and 1.3 mL of cocoamidopropyl hydroxysultainesolution followed by addition of 1.0 g each of AZDC and CHZ. The slurrywas divided into three portions of 12 g and each was placed in acentrifuge tube. To each slurry tube, a mixture of 5 g MgO, 1.0 g SHMPand variable amounts (0.3 g, 0.5 g and 0.75 g) of encapsulated potassiumpersulfate (labeled as EnCap KPXHT in FIG. 2) was added and vigorouslystirred with a spatula. All sets of experiments were allowed to continuefor at least 24 hrs at test temperature. The heat evolution due to gasgeneration and hydration/product formation was measured at differenttemperatures as described earlier. The results are shown in FIGS. 1 and2.

FIG. 1 shows that the set time could be controlled by SHMP as a setretarder. FIG. 2 shows that when the composition included gas generatingcomponents, two distinct heat evolution events took place, one of whichwas due to the exothermic gas generation and the other was due to cementsetting. The exothermicity of the gas generation reduced theeffectiveness of the retarder to provide longer set times, due to highereffective temperatures reached during the gas evolution (FIG. 2). Theresults shown in FIG. 2 also indicated that the gas evolution took placewithin the first 10-15 minutes at the reaction temperatures. In all setsof reactions performed, the slurries were soft to touch at the time ofsetting, but became hard set with high degree of stiffness in 24 hrs.

Other retarders were tested at 140° F. to compare the set times withSHMP in the presence of gas generating compositions. These retardersincluded sodium citrate, sodium borate (borax) and amino tri(methylenephosphonic acid) pentasodium salt (Na₅ATMP). Sodium borate and sodiumcitrate were added at 20% by weight of MgO, and Na₅ATMP was added at 16%by wt of MgO. In all cases, the generation was complete in 30 minutes,as evident from the exothermic peak in heat evolution measurements. Theset time was 5.2 hrs for sodium borate and longer than 6 hrs for theother two retarders. The 22 hr samples were hard set in the case ofsodium borate and sodium citrate, but were soft set in the case ofNa₅ATMP.

Example 6—Cement Compositions Containing Viscosifiers

In the absence of viscosifiers, the foam structure of the set solids wasnot uniformly forming alternate gas and solid layers. When viscosifierswere present, a uniform and stable foam structure was observed.Additionally, the solids with low solubility in water, such as magnesiumoxide and AZDC, suspended well without settling.

In the preceding examples, 0.8% xanthan and HEC-VP solutions wereemployed as the mix fluids to prepare the compositions. The HEC-VPsolutions were acidic (pH<2) due to the method of preparation, whereasthe xanthan solutions were of neutral pH. It was found that when AZDCand an amine activator (e.g., carbohydrazide) were present together inxanthan or guar solutions, irrespective of the other components, gasgeneration was taking place prematurely, whereas in HEC-VP solutionsthere was no gas generation until the addition of magnesium oxide.Depending on the desired timing of gas generation and the desiredsolvent, a process modification that requires keeping AZDC and amineactivator may be necessary. Alternately, when carbohydrazide was used incombination with an oxidizer as the AZDC-free gas generatingcomposition, any viscosifier could be used without premature gasgeneration problems.

OTHER EMBODIMENTS

It is to be understood that while the invention has been described inconjunction with the detailed description thereof, the foregoingdescription is intended to illustrate and not limit the scope of theinvention, which is defined by the scope of the appended claims. Otheraspects, advantages, and modifications are within the scope of thefollowing claims.

What is claimed is:
 1. A method of treating a lost circulation zonefluidly connected to a wellbore, the method comprising: a) forming afoamed cementitious composition comprising: magnesium oxide (MgO); asalt selected from the group consisting of magnesium chloride (MgCl₂),magnesium sulfate (MgSO₄), ammonium hydrogen phosphate (NH₄H₂PO₄), andhydrates thereof; a hydrazide or a semi-carbazide; an oxidizer; and afoam surfactant; and b) introducing the foamed cementitious compositioninto the lost circulation zone.
 2. The method of claim 1, wherein theoxidizer is selected from the group consisting of peroxide, persulfate,percarbonate, perbromate, perborate salts of ammonium, alkali earthmetals, and alkaline earth metals.
 3. The method of claim 1, wherein theweight ratio of the hydrazide or semi-carbazide to the oxidizer is about1:0.25 to about 1:5.